Molecular assemblies, such as a monomolecular film (monolayer) having a molecular arrangement or built-up film (monolayers=multilayer) formed by building up a plurality of the monomolecular films, are widely employed as materials for electronics devices, materials for surface protection, hyper-filtration membrances which utilize gaseous molecules or ion-permselectivity, functional thin films for sensors and permeability-controlling films for material delivery by utilizing the ultra-thinness and denseness thereof.
The Lamgmuir-Blodgett process (LB process) is generally known as a method for building up a monomolecular film of an amphiphilic molecule formed at the gas liquid interface on a substrate. The range of use of various LB films prepared by this method has been increased in recent years (see, Solid Physics 17 (12) 45 (1982)).
The molecular assemblies comprising an LB film exhibit various functions due to the molecular orientation and ultra-thinness. However, they have disadvantages in that they are physically delicate and the film structure is liable to be broken, or they have many structural defects depending upon the compounds employed and a high density can not be obtained.
In order to solve the problem of providing a film which has a uniform structure with an excellent packing of molecules, the film structure of the molecular assembly is required to be physically strengthened.
One of effective means for physically strengthening the film structure of the molecular assembly is crosslinking or polymerization.
With regard to the polymerization of molecular assemblies, such as an LB film and liposome, conventional polymerizable compounds employed and their polymerization are summarized in H. Bader et al., Advances in Polymer Science, Vol. 64, page 1 (1985) and R. Buschl, et al., Macromol. Chem. Suppl., Vol. 6, page 245 (1984).
The study of polymerizable amphiphilic compounds had been active in the 1980's. Widely used methods employ unsaturated compounds such as vinyl, diene and diacetylene compounds. The unsaturated bonds are cleaved by ultraviolet light (UV) or a radiation, such as gamma rays, so as to carry out polymerization. However, these methods have difficulty in keeping the order of molecular arrangement by distortion due to polymerization after the cleavage of the unsaturated bonds, though fast polymers can be obtained.
The orientation of the film is greatly influenced by the lengths of the alkyl chains and the type of the terminal hydrophilic group, as pointed out in A. Laschewsky and H. Ringsdorb, Mactomolecule, Vol. 21, page 1936 (1988). Hence, compounds providing a polymerized film having good orderliness are limited to a small number of compounds.
There is disclosed in A. Laschewsky, J, Am. Chem. Soc., Vol. 109, page 788 (1987) that in the amphiphilic compounds having various unsaturated bonds useful for radiation polymerization, the polymeric groups are carried through spacer groups so as to maintain order.
JP-A-57-159506 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") discloses that the polymer films of the monomolecular films of unsaturated compounds (surfactants) and built-up films thereof, prepared by radiation polymerization are used as hyper-filtration films.
Conventional methods for polymerizing these compounds having unsaturated bonds by radiation have the following problems. First, turbulence in the arrangement structure or the disordered agglomeration or precipitation of molecules is liable to be caused by polymerization. Hence, the specific molecular design, for example, the introduction of spacer groups into molecules must be made to prevent such a problem arising. Second, irradiation with ultraviolet light or gamma rays poses a problem in that additives often coexisting with polymerizable amphiphilic molecules are decomposed or denatured. Third, films prepared by such a polymerization have very poor compatibility with organisms and their applications to the tissues of organisms as permeability-controlling films for medicines are limited.
Methods for forming a disulfide bond by the oxidative polymerization of dithiols are proposed in J. Am. Chem. Soc., Vol. 109, page 4419 (1987) as polymerization methods which do not require radiation. Further, methods for radical polymerizing the above-described compounds having unsaturated bonds in the presence of initiators are also useful.
In these methods, however, the initiators must be used during polymerization. Hence, there must be required a stage for removing the initiators from the film-forming system after the completion of polymerization. In addition, these methods have a problem in that co-existing materials are affected by the initiators, including oxidation-reduction agents.
To improve compatibility with organisms by improving polymerization forms, methods for self-condensation-polymerizing the molecular films of long-chain alkyl derivatives of amino acids are disclosed in Macromol. Chem. Rapid Commun., Vol. 3, page 167 (1982) and Thin Solid Films, Vol. 133, page 39 (1985), and a method for condensation-polymerizing similar derivatives in the presence of carbodiimide as a condensing agent is disclosed in J. Am. Chem. Soc., Vol. 108, page 487 (1986). However, the condensation reaction in the self-condensation polymerization of these methods proceeds very slowly, and the method using a condensing agent has a problem in that the condensing agent and by-products are left behind and the condensation reaction is hard to handle. This is because the efficiency of the condensation reaction must be controlled.
Methods for preparing monomolecular films by using optical active amino acids and ester derivatives thereof are described in J. Am. Chem. Soc., Vol. 111, page 1115 (1989) and ibid., Vol. 111, page 1436 (1989). There is suggested that the molecular arrangement films of these optical active materials give excellent orientation and denseness. However, no method is disclosed therein for strengthening the structure thereof by rapidly self-polymerizing the films of these amino acid derivatives so as to prepare a thin film of an optically active polypeptide.